The present invention is directed to the use of laser light in integrated-circuit fabrication. It is directed particularly to severing conductive links in otherwise completed electronic circuits so as to indicate which parts of the circuit are defective and cannot be used and which spare parts of the circuit are to be used in their stead.
The designs of some large semiconductor-memory chips take into account the fact that the high number of memory cells makes a fabrication defect likely. To reduce the resultant waste, these designs include extra cells in the form of spare rows and columns that can be brought into use if a standard row or column is found to be defective. If testing reveals that a row or column is defective, the manufacturer vaporizes certain conductive links in the chip, thereby removing the defective row or column from service and programming the chip to replace the removed row or column with one of the spares.
The chip 10 of FIG. 1 shows in simplified form a possible arrangement of one of the conductive links used for encoding. Formed on a thick substrate 12 of single-crystal silicon might be a thin layer 14 of silicon dioxide. A conductive link 16 made, for instance, of a tungsten or molybdenum layer 18 bonded by sintering to a polysilicon layer 20 may be disposed between layer 14 and one or more oxide layers 22.
To break the link 16, a laser beam is focused through the layers 22, which are essentially transparent to the laser-beam wavelength employed, and onto the link, which absorbs the laser-beam energy and vaporizes, rupturing the layers above it to allow the erstwhile link material to escape.
Successful application of this technique depends greatly on various process parameters as well as on the particular chip design. Excessively intense laser radiation at isolated locations, for example, can result in damage to surrounding material, such as the thin oxide layer 14, and could thus result in operational defects. On the other hand, insufficient beam intensity or duration tends to permit flashing that causes undesirable remaining continuity in the link.
For this reason, considerable effort has been directed to so adjusting various process variables as to sever links reliably. One such process variable is the beam's intensity profile. Ordinarily, the intensity profile of a laser beam is Gaussian as a function of distance from the center of the beam. This profile is not optimum for link severing, however, because a significant length of the link needs to be vaporized; if the skirt regions of the Gaussian curve are to be counted on to provide the necessary vaporization energy, the center, peak region of the beam may deliver excess power at a localized region in the link and thus cause damage to the chip. This effect is exacerbated by the nonlinear nature of the heating process: the hotter the link material becomes, the more efficient it is at absorbing energy from the beam. On the other hand, if only the central, maximum region is counted on to deliver the requisite vaporizing intensity, then the beam has to be widened to such an extent that the total laser power required for a given intensity is undesirably high.
For these reasons, efforts have been made to make beam intensity more uniform. In one link-vaporizing arrangement, for instance, the beam is directed to an aperture through which only the central part of the beam can pass. To some extent, this approach does contribute to intensity uniformity. But it suffers from the inefficiency that arises from the fact that the power in the outer parts of the Gaussian curve is wasted. Moreover, I have found that such an approach tends to result in "hot spots" that result from diffraction that the aperture causes.